This invention is related to concrete and more particularly to structural grade lightweight concrete especially suited for construction of ocean structures.
Normal weight concrete has found considerable application in energy-related offshore structures, such as oil drilling and production platforms. Proposals abound for other applications, such as submerged oil production enclosures, seafloor fuel storage tanks, and even liquefied natural gas transport ships. In any one of these applications, a construction material lower in unit weight than normal weight concrete would be beneficial to the designer in planning a structure of less draft or higher payload capacity.
Structural lightweight aggregate concrete, using naturally occurring or manufactured lightweight aggregate, is a commonly used construction material. Typically lightweight aggregate particles have a void volume of 30 to 60 percent whereas natural "hard rock" aggregates have a void volume of about five percent or less. The greater volume of entrapped air in lightweight aggregate is the reason for the reduced weight compared to normal weight (hard rock) concrete.
Regular lightweight concrete materials present a problem when applied to ocean applications, such as buoyancy tanks, because seawater permeates through the Portland cement binder and eventually fills the voids within the aggregate particles. Hence the unit weight of the concrete changes with time. This is undesirable because the actual buoyancy of the structure at any given time is not known with certainty. Regular lightweight aggregate concrete can increase in weight by as much as 25 percent from seawater absorption.
Another problem with regular lightweight concrete is that the maximum compressive strength is limited by the strength of the aggregate particles. In some locations of the country, lightweight concrete has a maximum compressive strength of 6000 psi whereas in other locations only 5000 psi. The maximum strength depends on the properties of the lightweight aggregate available at the different locations.
Prior art patents frequently have used polymeric particles, polystyrene-bead materials, or other lightweight cellular materials in place of at least some of the aggregate in concrete to reduce the density. However, none of the prior art lightweight concretes have satisfactory compressive strength for use with ocean structures due to the limited strength of the aggregate particles used.
This invention provides a means to minimize seawater absorption by the aggregate; and also provides a means to produce concretes having increased compressive strength compared to regular lightweight concretes of equal unit weight.